A variety of methods have been proposed for the remediation of soil containing organic contaminants. Many of the proposed methods involve excavation and subsequent incineration of soil or thermal desorption of soil contaminants with the attendant difficulties of treatment and/or disposal of off-gases. A major detriment to such processes, however, is the cost of excavating and transporting the soil, which results in a total cost that may approach 1,000 U.S. dollars per ton of soil.
To avoid at least a portion of these costs, several types of in-situ heating processes have been proposed to volatilize and/or decompose soil contaminants. These include vitrification of the soil by electrode heating, steam or hot air heating of the soil through an auger system or through stationary pipes, and radio-frequency or electrical heating of the soil by means of a surface heater.
Brouns et al, U.S. Pat. No. 4,376,598, disclose a vitrification process in which the soil is heated to approximately 1500.degree. C. At or about this temperature the soil forms a glass-like mass which traps the contaminants and any decomposition products therein. This process is, in reality, a stabilization process rather than a decontamination process since the soil undergoing treatment has lost its physical and chemical identity. Both an auger system for injecting steam or hot air and a process for steam injection through stationary pipes have been practiced commercially. These methods have a limited use, primarily in the decontamination of soil containing small areas of deep contamination such as localized spills or leakages at service stations, storage tanks, pumps, and loading facilities. These methods are not as useful when applied to large areas of contaminated soil.
Bridges et al, U.S. Pat. No. 4,670,634, disclose an in-situ thermal process where the thermal energy is supplied by radio-frequency heating. This process is particularly applicable to water-containing soils where the steam generated in the soil serves to strip the organic contaminants from the soil. A somewhat related process is diclosed by copending U.S. patent application Ser. No. 427,418, filed Oct. 27, 1989 now U.S. Pat. No. 4,984,594, wherein the thermal energy is supplied by a relatively flat heater deployed at the surface of the soil. In this latter process, a lowered pressure is applied at the surface of the soil to remove vapors generated within the soil. This method is somewhat inefficient since significant vapor flow takes place largely near the surface of the soil or within a "blanket" placed on the surface of the soil which is more premeable to vapor flow than the soil it covers. It would be of advantage to provide a more effective method of collecting and moving from heated soil the vapors formed when soil contaminated by organic or semi-volatile inorganic contaminants is heated.